Sonodynamic therapy is a promising new modality for cancer treatment based on the synergistic effect on cell killing by the combination of a drug (typically a photosensitizer) and ultrasound. Umemura and co-workers have proposed that the mechanism of sonodynamic action consists of the excitation of the photosensitizer by sonoluminescent light (the short pulses of light emitted during the collapse of cavitation bubbles) with subsequent formation of singlet oxygen. An understanding of the mechanism of drug/ultrasound synergism is essential for possible further therapeutic applications of sonosensitizing therapy in tumor treatment. Electron paramagnetic resonance (EPR) was used to investigate the aqueous sonochemical reactions of the most effective sonodynamic agent, gallium- porphyrin derivative (ATX-70). Our studies of the effect of temperature and gas composition (N2/O2 and N2/Argon mixtures) were not compatible with the singlet oxygen mechanism of Umemura and co-workers. Instead, we found that the surfactant cetyl-trimethylammonium bromide could mimic the effect of ATX-70. It should be noted that the surfactant properties of the porphyrin ring in ATX-70 are enhanced by the presence of two attached C-10 chains. Our current hypothesis of the role of surfactants in sonodynamic cell killing involves the formation of peroxyl radical intermediates. The mechanism of the sonosensitized cell killing by non-toxic concentrations of N,N-dimethylformamide, N-methylformamide, and dimethylsulfoxide was investigated by EPR and spin-trapping using the spin trap 3,5-dibromo-4- nitrosobenzene sulfonate in nitrogen-saturated aqueous solution. In air- saturated aqueous solutions the carbon-centered radicals were converted to the corresponding peroxyl radicals and spin trapped with 5,5-dimethyl-1- pyrroline-N-oxide (DMPO). These radical species by virtue of their longer lifetimes and higher selectivity, compared to OH radicals which are also formed in sonicated solutions, are the species responsible for sonodynamic cell killing with dimethylformamide, monomethylformamide, or dimethylsulfoxide. The effective temperature in collapsing cavitation bubbles in multi-bubble field was measured by EPR-spin trapping during the sonolysis of H2O/D2O mixtures using phenyl-t-butyl nitrone-type spin traps. From the semiclassical model of the temperature dependence of the kinetic isotope effect for H and D formation, we estimated the effective temperature in the cavitation region in which H and D atoms are formed by the ultrasound-induced pyrolysis of water molecules to be in the region of 2000-4000 K.